Motorola 24-Bit Digital Signal Processor, DSP56000 manual Summary of Pipeline-Related Restrictions

NORMAL PROCESSING STATE

The DO instruction is another instruction that begins execution during the decode cycle of the pipeline. As a result, there are a number of restrictions concerning access conten- tion with the program controller registers accessed by the DO instruction. The ENDDO instruction has similar restrictions. APPENDIX A - INSTRUCTION SET DETAILS con- tains additional information on the DO and ENDDO instruction restrictions.

Case 5: A resource contention problem can occur when one instruction is using a register during its decode while the instruction executing is accessing the same resource. One ex- ample of this is as follows:

MOVEC	X:$100,SSH
DO	#$10,END

The problem occurs because the MOVEC instruction loads the contents of X:$100 into the system stack high (SSH) during its execution cycle. The DO instruction that follows pushes the stack (LA → SSH, LC → SSL) during its decode cycle. Therefore, the two instructions try writing to the SSH simultaneously and conflict.

7.2.2 Summary of Pipeline-Related Restrictions

The following paragraphs give a summary of the instruction sequences that cause pipe- line effects. Additional information about the individual instructions can be found in

APPENDIX A - INSTRUCTION SET DETAILS.

DO instruction restrictions:

The DO instruction must not be immediately preceded by any of the following instruc- tions:

BCHG/BCLR/BSET LA, LC, SSH, SSL, or SP

MOVEC/MOVEM to LA, LC, SSH, SSL, or SP

MOVEC/MOVEM from SSH

The DO instruction cannot specify SSH as a source register, as in the following example:

DO SSH,xxxx

Restrictions near the end of DO loops:

Proper DO loop operation is guaranteed if no instruction starting at address LA-2, LA-1, or LA specifies the program controller registers SR, SP, SSL, LA, LC, or (implicitly) PC as a destination register, or specifies SSH as a source or a destination register.

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